Implement Slice A - Enhanced User Registration & Profile

[Tyrrnien81]

📝 Pull Request: Implement Slice A - Enhanced User Registration & Profile
🎯 Summary
This PR implements Slice A of the Hybrid Vertical Slice development plan, completing the user journey: "As a user, I want to provide my full name during signup and see it reflected throughout the app"
🚀 What's Changed
Database Layer
✅ Added firstName and lastName fields to User model
✅ Created database migration with backward compatibility (default empty strings)
✅ Updated Prisma schema with proper field mapping (first_name, last_name)
Backend API
✅ Enhanced signup validation schema to require name fields (1-50 character limit)
✅ Updated /auth/signup endpoint to handle firstName/lastName
✅ Modified /auth/login response to include user names
✅ Maintained existing JWT token structure and authentication flow

Frontend Experience
✅ Added First Name and Last Name input fields to SignupForm component
✅ Updated form validation to require name fields before submission
✅ Enhanced User interface in TypeScript with name properties
✅ Updated auth store and API service signatures
✅ Implemented real user name display in Dashboard welcome message
✅ Enhanced Account section with actual user data (read-only fields)
Testing & Quality
✅ Updated E2E tests to include name field validation
✅ Added test verification for Dashboard name display
✅ All existing authentication flows remain functional
✅ Form validation and error handling working correctly
🧪 Testing Results
✅ Manual Testing: Successfully created account with names "Leo Jeong" and verified display
✅ Dashboard Integration: Welcome message shows "Welcome back, Leo Jeong!"
✅ Account Page: All user data fields populated correctly
✅ Form Validation: Name fields required, proper error states
✅ API Endpoints: Signup/login working with extended data structure

🎯 User Journey Completed
✅ User visits signup page
✅ User enters first name, last name, email, and password
✅ Form validates all required fields
✅ Account created with name data stored
✅ User redirected to dashboard with personalized welcome
✅ User name displayed consistently throughout app
✅ Account section shows complete user profile
🔄 Backward Compatibility
✅ Existing users get empty string defaults for names
✅ All existing API endpoints remain functional
✅ No breaking changes to authentication flow
✅ Database migration handles existing data gracefully